1. Field of the Invention
The present invention relates to a halogen-free flame-retardant polyolefin-based resin composition improved in low-temperature resistance (cold weather resistance). More specifically, it relates to a halogen-free flame-retardant polyolefin-based resin composition which is free from a surface whitening phenomenon and which is improved in flame retardancy and melt index and is also improved in low-temperature resistance. Further, the present invention relates to an expansion-molded article of the above resin free of large-sized gas bubbles.
2. Description of Related Art
Polyolefin resins are easily combustible themselves. It is therefore necessary to impart polyolefin resins with flame retardancy for preventing various calamities caused by fire, etc., and a variety of proposals have been hitherto made. One proposal is concerned with a flame-retardant polyolefin-based resin composition obtained by incorporating an organic halide or a combination of an organic halide and antimony trioxide into a polyolefin resin.
However, the problem of the above resin composition is that it corrodes a molding machine when molded. Further, it generates a large volume of smoke in fire, and the smoke is toxic and corrosive. For overcoming these problems, there has been proposed a method in which a flame-retardant polyolefin-based resin composition is provided by incorporating a large amount of a halogen-free safe flame-retardant such as magnesium hydroxide or aluminum hydroxide into a polyolefin resin. This method is disclosed in many publications such as JP-A-50-119848, JP-A-53-12943, JP-A-54-77658, JP-B-57-10898, JP-A-60-243155 and U.S. Pat. No. 4,396,730.
A polyolefin-based resin composition obtained by incorporating a large amount of metal hydroxide into a polyolefin resin satisfies the practical use level requirements of mechanical strength at room temperature (ordinary temperature), and causes almost no problem when used in a warm environment. However, this polyolefin-based resin composition has a problem on low-temperature resistance since it shows a great decrease in mechanical strength, particularly impact strength, when used at a low temperature. That is, the polyolefin-based resin composition has a problem in that it is embrittled in a low-temperature environment so that the use thereof in such an environment is unacceptable or impossible.
The term "low-temperature resistance" in the present specification specifically means the following. A resin composition having "low-temperature resistance" refers to a resin composition which shows almost no decrease in impact resistance such as Izod impact strength and du Pont impact strength when used at a low temperature such as a temperature below the freezing point, or which shows a sufficiently low brittle temperature in a brittle temperature test. A material having no low-temperature resistance is embrittled and unsuitable as a material for use in a low-temperature environment such as a cold district or an extremely cold district.
However, the above publications on flame-retardant polyolefin-based resin compositions do not recognize anything concerning the need for of improvement in low-temperature resistance, and naturally, those publications describe nothing concerning low-temperature resistance. The properties of a polyolefin-based resin composition differ depending upon whether or not a metal hydroxide incorporated as a flame retardant is surface treated. The metal hydroxide which is not surface-treated has poor compatibility with a polyolefin resin so that it cannot be homogeneously dispersed in the resin. As a result, a mixture of these shows a low melt index, and a molded article of the mixture (composition) is very poor in toughness and low-temperature resistance and flame retardancy. Further, when the metal hydroxide is other than aluminum hydroxide, a composition of these shows a high surface whitening phenomenon.
On the other hand, a surface-treated metal hydroxide not only has excellent compatibility with a polyolefin resin but also has excellent dispersibility in polyolefin resin. Therefore, a mixture of a surface-treated metal hydroxide and a polyolefin resin has a greatly improved melt index, and a molded article thereof has excellent toughness at room temperature. Further, the surface whitening phenomenon of a molded article of a mixture containing a surface-treated metal hydroxide other than aluminum hydroxide and a polyolefin resin is considerably inhibited when some surface-treating agents are selected. However, the low-temperature resistance and flame retardancy of these compositions is still at low levels.
Further, the expansion molding method has a problem in that large-sized gas bubbles occur so that it is difficult to obtain an expansion-molded article having uniform and fine gas bubbles.
JP-A-50-119848 discloses a self-distinguishing resin composition which is improved in melt index for easy injection molding, by incorporating, as a lubricant, a salt of a fatty acid having 8 to 20 carbon atoms and a metal such as aluminum, zinc, magnesium or calcium into a self-distinguishing polyolefin-based resin composition containing magnesium hydroxide. However, JP-A-50-119848 mentions nothing concerning the surface treatment of the magnesium hydroxide, nor does it describe anything concerning the low-temperature resistance of the self-distinguishing resin composition as an end product. The self-distinguishing resin composition actually has a very low level of low-temperature resistance, and its flame retardancy is also poor.
JP-A-53-12943 discloses that a polyolefin-based flame-retardant resin composition of which the flame retardancy, toughness and injection moldability are well-balanced and excellent can be obtained by incorporating metal soap as a lubricant and an alkali metal salt of an organic carboxylic acid as a flame retardant aid into a flame-retardant polyolefin-based resin composition containing magnesium hydroxide. However, JP-A-53-12943 does not mention anything concerning the surface treatment of the magnesium hydroxide, nor does it describe anything concerning the low-temperature resistance and the surface whitening phenomenon of the polyolefin-based flame-retardant resin composition. This resin composition actually has the following problems. It has poor low-temperature resistance, the surface whitening phenomenon is intensified by adding an alkali metal salt of an organic carboxylic acid, and a molded article obtained therefrom is degraded in appearance. JP-A-53-12943 uses Izod impact strength values according to ASTM D-256 for toughness evaluation. ASTM D-256 is a method in which Izod impact strength is measured at a temperature of 23.degree. C..+-.2.degree. C. A flame-retardant polyolefin-based resin composition according to JP-A-53-12943 shows a high Izod impact strength value at room temperature, while it shows a very low one at a temperature below the freezing point. Further, the alkali metal salt of an organic carboxylic acid does not show the flame retardant aid effect as much as that described in JP-A-53-13943. The melt tension of the flame-retardant polyolefin-based resin composition is very low. As a result, strands thereof easily break when processed with a resin processing machine such as an extruder, and it is very difficult to continuously process it with a pelletizer.
U.S. Pat. No. 4,396,730 discloses a flame-retardant thermoplastic resin composition containing magnesium hydroxide surface-treated with an alkali metal salt of oleic acid as a flame retardant and a magnesium oleate or aluminum oleate as a flame retardant aid. U.S. Pat. No. 4,396,730 seeks to provide a resin composition which has excellent moldability and gives a molded article having improved flame retardancy, an excellent appearance and excellent strength. However, it pays no attention to the surface whitening phenomenon. That is, the surface whitening phenomenon cannot be avoided when an alkali metal salt of oleic acid is used, and the alkali metal salt of oleic acid is nevertheless used for surface-treatment of magnesium hydroxide. For this reason, the resin composition of U.S. Pat. No. 4,396,730 has a defect in that a molded article therefrom suffers a surface whitening phenomenon to a great extent. Further, U.S. Pat. No. 4,396,370 describes nothing concerning the low-temperature resistance of the resin composition.
JP-A-54-77658 discloses a resin composition obtained by incorporating polyvinyl acetate or an ethylene-vinyl acetate copolymer as a flame retardant aid into a flame-retardant olefin polymer containing magnesium hydroxide. However, JP-A-54-77658 describes nothing concerning the low-temperature resistance. The resin composition has an effect on the inhibition of glowing to some extent, but shows no improvement in low-temperature resistance.
JP-B-57-10898 discloses a resin composition obtained by incorporating a carbon powder such as carbon black, as a flame retardant aid, into a self-distinguishing thermoplastic resin composition containing magnesium hydroxide. The disclosed resin composition shows some improvement in flame retardancy, but shows no improvement in low-temperature resistance. Further, JP-B-57-10898 describes nothing concerning the low-temperature resistance.
JP-A-60-243155 seeks to achieve excellent moldability and excellent appearance, physical properties and flame retardancy of a molded article by incorporating a specific magnesium hydroxide into a thermoplastic resin. The specific magnesium hydroxide refers to a magnesium hydroxide having excellent dispersibility and non-aggregation properties and having a BET specific surface area of 20 m.sup.2 /g or less and a BET specific surface area/Blaine permeability method specific surface area ratio of 1.about.3. However, JP-A-60-243155 does not at all mention the low-temperature resistance, and no composition according to JP-A-60-243155 is satisfactory in low-temperature resistance.
As explained above, no prior art documents seek to improve the low-temperature resistance, nor are the compositions according to the prior art documents satisfactory in the improvement of low-temperature resistance. For providing a flame-retardant polyolefin resin composition practically suited for use in a low-temperature environment, it is required to accomplish all of low-temperature resistance, surface whitening resistance, flame retardancy and an adequate melt index at the same time.
In the expansion-molding method, further, it is required to provide an expansion-molded article having uniform and fine gas bubbles without large-sized gas bubbles. The large-sized gas bubbles decrease the strength and heat insulation of the resin.